The present invention relates to a contact lens or, more particularly, to a contact lens made of a novel plastic material having improved properties such as oxygen permeability.
As is generally understood, plastic-made contact lenses used for correction of visual acuity must satisfy several requirements, for example, that the lens has no adverse influences on the metabolic function of the wearer's cornea with good permeability to oxygen, that the plastic has a high clarity to exhibit satisfactory effects of visual acuity correction, that the lens gives no damage to the cornea, that the surface of the lens is resistant against stain by the ingredients contained in the tear fluid, that the plastic material can be freely and precisely shaped into an optimum form to ensure smooth moving thereof on the cornea, that the plastic material is not excessively adherent to the surface of the cornea, that the lens can be easily handled with little danger of inadvertent breaking and so on.
Contact lenses made of a plastic resin currently on use are classified into hard contact lenses, the diameter of which is usually smaller than that of the cornea, and soft contact lenses, the diameter of which is usually larger than that of the cornea, according to the nature of the plastic material. None of the plastic materials used for shaping these types of contact lenses in the prior art, however, can satisfy all of the above mentioned various requirements. For example, poly(methyl methacrylate) resin, which is a typical plastic material for shaping hard contact lenses, has a problem in the low permeability coefficient to oxygen of about 0.005.times.10.sup.-9 cm.sup.3 (STP).multidot.cm/cm.sup.2 .multidot.sec.multidot.cmHg to cause inhibition of the air respiration of the wearer's cornea which is a tissue void of blood vessels. Therefore, continued wearing of such a contact lens over a length of time is ophthalmically very undesirable. A conventional measure undertaken to mitigate this disadvantage is to design the contact lens with a relatively large base curvature to ensure good mobility of the lens on the cornea so as to accelerate exchange of the oxygen-containing tear fluid. This lens design is, however, not always sufficient to solve the problem in the continued wearing of the lens. In addition, the high mobility of the lens is necessarily accompanied by an increased danger of dropping of the lens from the eye, for example, by a strong blinking of the eyelids and unpleasant feeling of the wearer caused by touching of the lens to the upper and lower eyelids.
Several proposals and attempts have been made to increase the oxygen permeability of contact lenses made of a poly(methyl methacrylate) resin according to which the resin is modified by the copolymerization of methyl methacrylate with certain organosilicon compounds (see, for example, Japanese Patent Kokai Nos. 50-87184 and 54-55455). Such a copolymeric poly(methyl methacrylate) resins are still insufficient in respect of the oxygen permeability which is far from giving full supply of the oxygen physiologically required by the cornea through the lens.
In view of the above described problems of hard contact lenses in respect of the oxygen permeability and wearer's feeling, soft contact lenses have been developed using a relatively flexible plastic material which is typically a poly(2-hydroxyethyl methacrylate) of a hydrated gel type or a transparent silicone rubber of a non-hydrated elastomer type. The former material, however, has several disadvantages that the material is less suitable for the visual acuity correction than the hard contact lenses made of a poly(methyl methacrylate) resin, that the material is mechanically not strong enough to become sometimes damaged, that the material as a hydrated gel is susceptible to stain by the tear fluid and uptake of the chemical ingredients containing in the cleaning solution and that the oxygen permeability of the material is not always sufficiently high to satisfy the oxygen demand of the cornea. The latter material of transparent silicone rubbers has problems in the exact shaping works into the desired lens form as a natural consequence of the elastomeric nature thereof and in the ready deposition of the constituents in the tear fluid inside the lens although the material is advantageous in respect of the high oxygen permeability.